HDLC does not perform Error Recovery......?

aljugan

Dear Sirs.......i have one question about the Chapter 4 of Cisco Press...."Fundamentals of Wan"...in page 90 of this book its written that HDLS does not perform Error Recovery. my question is this that HDLS is data link layer protocol and on the top in Transmission controll protocol there would be TCP or UDP am i right ?. so does HDLC use udp ?. because as i read in previous chapter udp does not perform error recovery but tcp performs this alway....so in short my question is if its written in the book that HDLC does not perform error recovery so which protocol is used by it on transmission layer....Best Regards.

networker050184

Two different layers. You can use TCP or UDP (Layer 4) over HDLC (Layer 2).

theodoxa

Here's an example (accessing a picture online):

Layer 7 (Application) -- HTTP
Layer 6 (Presentation) -- JPEG
...
Layer 4 (Transport) -- TCP (Port 80)
Layer 3 (Network) -- IPv4
Layer 2 (Data Link) -- HDLC
Layer 1 (Physical) -- T1

PCHoldmann

Lower level protocols that don't have error detection rely on upper layers to provide it. In the case of FTP running over TCP running over PPP, a packet that is corrupted over a PPP link would get detected there, and dropped. TCP would still have to handle the re-send, but you don't transmit corrupted packets any further over the network. With HDLC would not be detected until TCP was unwrapping it at the other end. With something running over UDP and HDLC, the packet would get to the actual client or server process at the other end before it is invalidated (or worse, used as-is). Of course the error checking comes with overhead, so it's a tradeoff where you want to spend the overhead.